trying to join 3 statements together in a select case - vba

Question

I am trying to join these 3 statements together. I can get two to work but not the third.
Check is looking to see if the 5th character = a,b,c,d.
Check0 is looking to see if characterS 2-9 are all numbers.
Check3 is looking to see if the first 3 characters are numbers.
Desired effect
Before > After
Check 0 e01730101.pdf > S-173-0101
Check3 173d00510.pdf > S-173-D005
Check e173d0061.pdf > S-173-D006
Right now Check0 is not working. When I run the code it seems to skip my Case 13 Check0 statement. Anyway i can write this so the 3 checks are not conflicting each other?
Option Explicit
Sub Convert()
Application.ScreenUpdating = False
Dim rng As Range, aCell As Range
Dim val As String, Check, Check0, Check3
Dim LastRow As Long
LastRow = Range("A" & Rows.Count).End(xlUp).Row
Set rng = Range("A2:A" & LastRow)
For Each aCell In rng.Cells
Select Case Len(aCell)
'Case 12 left out
Case 13
Check = Mid(aCell, 5, Len(aCell) - 12)
If Check = "a" Or Check = "b" Or Check = "c" Or Check = "d" Then 'Existing Standard
val = "S-" & Left(aCell, Len(aCell) - 13) & Mid(aCell, 2, Len(aCell) - 10) & "-" & Mid(aCell, 5, Len(aCell) - 9)
Check0 = IsNumeric(Mid(aCell, 2, Len(aCell) - 5)) '|||PROBLEM|||
ElseIf Check0 = True Then 'Existing Three Line Diagrams
val = "S-" & Left(aCell, Len(aCell) - 10) & "-" & Mid(aCell, 4, Len(aCell) - 9)
End If
Check3 = IsNumeric(Left(aCell, 3))
If Check3 = True Then 'Standard after page 9
val = "S-" & Mid(aCell, 1, Len(aCell) - 10) & "-" & Mid(aCell, 4, Len(aCell) - 9)
End If
Check = ""
Check0 = ""
Check3 = ""
'Case 14 left out
Case Else 'All other pages
val = "_Mod " & Left(aCell, Len(aCell) - 4)
End Select
val = UCase(val)
val = val & " " & aCell.Offset(, 2) & aCell.Offset(, 3)
aCell.Offset(, 1).Value = val
Next
Application.ScreenUpdating = True
End Sub

Answer 1

The issue is that your Check0 variable is not initialised when you check it. It would be best to first do all your three checks and only then perform the If...ElseIf....
Secondly, this expression is using the wrong parts of the input string:
val = "S-" & Left(aCell, Len(aCell) - 10) & "-" & Mid(aCell, 4, Len(aCell) - 9)
I don't really understand why you use Len(aCell) - something, since it is known that the length is 13. Certainly Left(aCell, Len(aCell) - 13) is quite useless, as it will be the empty string.
Here is a correction with some other optimisations (pos is a Long):
Case 13
Check = InStr("abcd", Mid(aCell, 5, 1))
Check0 = IsNumeric(Mid(aCell, 2, 8))
Check3 = IsNumeric(Left(aCell, 3))
pos = IIf(Check3, 1, _
IIf(Check, 2, _
IIf(Check0, 3, 0)))
val = IIf(pos, "S-" & Mid(aCell, pos, 3) & "-" & Mid(aCell, pos+3, 4), "??")